1. Field of the Invention
The present invention relates to centrifugation, and particularly to the sample retaining means used in connection with a centrifuge rotor.
2. Description of Related Art
The invention of OptiSeal.TM. centrifuge tubes (commercialized by Beckman Instruments Inc., U.S.A.) is disclosed in U.S. Pat. No. 5,127,895, entitled "SelfSeal Centrifuge Tube", assigned to the assignee of the present invention. OptiSeal.TM. tubes are thin-walled vessels which are sealed using plugs under forces developed by centrifuge operation. The particular geometry of the OptiSeal.TM. tube illustrated and described in the '895 patent has a hemispherical top portion around the tube stem. Such tubes are primarily designed for use in vertical tube rotors (in which axis of each tube cavity is parallel to the rotor spin axis) and near vertical tube rotors (in which axis of each tube cavity is at a small oblique angle on the order of 10.degree. to the rotor spin axis). A support cap or spacer is secured to a counterbore in the cavity and engages the top of the tube. The advantages of the spacer are that it supports against the top portion of the tube and the plug so that it prevents deformation of the tube top caused by centrifugally induced hydrostatic pressure and it provides support to seal the plug to the cube stem against the internal the hydrostatic pressure.
However, while the patented tube and closure have been found to be effective in maintaining a seal under high centrifugal forces when applied to a vertical tube rotor and near vertical tube rotors, the disclosed support spacer might not be adequate to prevent deformation of the tube near the tube stem when applied to fixed angle rotors and swinging bucket rotors. It has been found that the upper portion of the tube around the tube stem or opening of the tube may be deformed in fixed angle rotors (in which each rotor cavity is inclined at a large angle on the order of 25.degree. to the rotor spin axis), or buckle the tube in the axial direction in swinging bucket rotor applications (in which the centrifuge tubes are pivotally supported on the rotor to allow the axis of the tubes to swing outwards towards horizontal upon centrifugation). Tube deformation is more pronounced when there is significant amounts of air enclosed in the tube, from air either entrained in the liquid material or left in the tube because the liquid does not fill it as a result of limitation in the tube filling procedures.